Method of controlling the cut-off voltage of a cathode ray tube



G. J. MozEK 2,951,279 METHOD oF CONTROLLING THE CUT-OFF VOLTAGE oF A cATHoDE RAY TUBE Sept. 6, m0

Filed April l0, 1958 INVENTOR G'soacs Maza-K United States Patent lVIETHOD OF CONTROLLING THE CUT-OFF VOLTAGE F A CATHODE RAY TUBE George J. Mozek, West Paterson, NJ., assignor to 'Tung- Sol Electric Inc., a corporation of Delaware Filed Apr. 10, 1958, Ser. No. 727,654

1 Claim. (Cl. 29-25.13)

The present invention relates to electron guns for cathode ray tubes and comprises a novel method of precision construction which insures a high degree of control of the cut-ott voltage of the cathode ray tube in which the gun will ultimately be installed.

The conventional electron gun comprises a cathode cylinder the closed end of which is coated with electron emissive material, a No. 1 grid within which the cathode is insulatedly mounted with the closed coated end of the cathode spaced from the aperture in the grid and additional cylindrical grids axially disposed with respect to the cathode and No. 1 grid and kept in spaced relation by means of insulating rods which support the grids by means of radially disposed pins fused into the rods and welded to the grids. Much care is taken to provide accurate spacing between the various grids and cathode prior to the mounting thereof on the insulating rods. Nevertheless with prior methods of adjustment and control the cut-otfvoltage of the cathode ray tube in which the gun is ultimately installed may vary as must as 35 vol-ts from that desired.

In accordance with the present invention adjustment of the spacing between two or more grids or between the cathode and a grid may be made so precise and accurate that the cut-off voltage may be controlled within 8 volts. The invention is predicated on the fact that there is a direct relationship between the' direct capacitance of cathode to grid No. 2 (accelerating grid) and the cut-off voltage of the cathode ray tube in which the gun is installed. The invention takes advantage of this direct relationship and of the fact that relatively minute change in cathode to grid No. 2 spacing or in the position of the control grid results in a relatively large change in cut-olf voltage. In accordance with the preferred method of the invention the gun elements are assembled and mounted in conventional manner except that the cathode to grid No. 1 (control grid) spacing is made slightly oversize. The assembled gun is then mounted in a suitable jig, the capacity between the cathode and grid No. 2 is read on a suitable capacity measuring device and the cathode pushed or bent, as by dimpling, toward the grids until the capacity meter reading corresponds to that of the desired cut-olf voltage. Alternatively the grid No. 2 is pushed toward the assembly of cathode and grid No. 1, the pins supporting the grid No. 2 being suiciently yielding to permit the small amount of longitudinal movement of grid No. 2 necessary for precise adjustment of the spacing. Another alternative is to hold the cathode and grid No. 2 fixed and adjust the .position of the control grid.

For a better understanding of the invention reference may be had to the accompanying drawing of which Fig. y1 is a longitudinal sectional view, partly in elevation, of an electron gun supported in a jig and connected to a capacity measuring device for practice of the invention;

Figs. 2 and 3 are vertical sectional views taken on the lines 2 2 and 3 3 respectively of Fig. 1; and

Fig. 4 is an enlarged horizontal sectional view through the cathode, control grid, and part of grid No. 2 while held in the jig of Fig. 1.

In the drawing, an electron gun for a cathode ray tube is shown as comprising a cathode sleeve 2 mounted in a ceramic disc 4 within the grid cup 6 (No. 1 grid), an accelerating grid 8 (grid No. 2) and additional cylindrical grids 10. The insulating rods, which may be of porcelain or glass, are identiiied by the reference numeral 14 and the radially disposed pins which extend from the grids and are fused into the rods 14 are indicated at 16. The gun comprising the above identiiied'elements is supported within a metal box 18 which serves as an electrical shield and which is grounded to the ground connection of a capacity measuring device indicated diagrammatically at 20. Device 20 may be, for example, adirect interelectrode capacitancepbridge which makes measurements at 465 kc., and preferably one that reads capacitance to within .0001 auf. `A lead 22 is connected at one end to the cathode sleeve 2 and at its other end to a conductor 2'4 which is insulatedly brought out through the rear wall of the box 18. The other end of the conductor 24 is connected to the low voltage side of the meter 20. A similar conductor 26, insulatedly brought out through the rear wall of box 18, is connected,

to the high voltage terminal of the meter 20 and within the box makes contact with the No. 2 grid 8 of the gun. Rotatably mounted in one end wall of the box 18 is a drum 28 having a knurled hand wheel 30. A plunger 32 adapted to t within the cathode cylinder 2 and to engage the closed end thereof is threadedly mounted in an axial passage in the drum for Vaxial movement upon rotation of the drum. Means, such as a key 33 on the plunger and keyway 34 on the jig, prevent rotation of the plunger with the drum. As the cathode must be insulated from the box 18, the plunger 32 should be of insulating material or otherwise electrically insulated from the box. Conveniently the outer surface of the drum 28 may be provided with a scale 35 and mounted on the box wall for cooperation therewith there may be a pointer 36. Each of the remaining grids of the gun as well as the control grid 6 is conductably connected to the wall of the box so as to be at ground potential. With this arrangement the gun is completely shielded electrically from the outside and the cathode and grid No. 2 are capacitatively coupled only through the aperture of the No. 1 grid. Starting with an oversize spacing between the closed end of the cathode cylinder 2 and the opening 38 in the No. 1 grid the drum 28 is turned to force the plunger 32 against the closed end of the cathode and thereby bring it nearer to the opening 38 in the grid 6. When the reading of the meter 20 is that corresponding to the desired cut-off voltage the plunger 32 is retracted and the gun is ready for mounting in the cathode ray tube.

The following specic values are given to illustrate the relation of cut-off voltage to direct capacitance between cathode and No. 2 grid.

A gun in which the designated capacitance was .1410 ggf. had a cut-ott voltage of 40 volts, whereas, when the electrode spacing of the same gun'was changed to yield a direct capacitance between cathode and No. 2v grid of .1445 auf. the cut-off voltage changed to 46 volts. Thus a change of about .0006 auf. corresponded to about l volt change in cut-olf voltage.

During rotation of the drum to urge the plunger against the cathode, the remaining parts of the gun should, of

course, be firmly held against longitudinal movement.v

construction illustrated, may comprise suitable projections v on the nonconductive support 40 of the jig which extend into the space between adjacent grids.

As heretofore indicated, instead of adjusting the spacing between the cathode and theNo. 2 grid by movement of the cathode' toward the No. 1 grid, as just described, the same result may be obtained by holding fixed therNo. lgrid and cathode assembly and forcing the No. 2 grid toward the No. 1 grid until the direct capacitance between the No. 2 grid and cathode is that corresponding to the desired cut-oti voltage. When this alternative method of control is employed the spacing between the No. l and No. 2 grids is made slightly oversize when the grids are assembled and mounted on the insulating rods.

As the cathode and No. 2 grid are capacitatively coupled only through the aperture in the control grid so that a shift in position of the control grid changes the cathode to No. 2 grid capacitance, the cut-oi'Ir yvoltage may also be controlled by so moving the control grid while holding fixed the cathode to No. 2 grid spacing.

From the foregoing description it will be apparent that the method of the invention provides for accurate conrtol of cut-off voltage of cathode ray tubes. It will also be apparent that the novel method of the invention may be conveniently practiced with a minimum of equipment.

The following is claimed:

In the manufacture of a cathode ray gun having a plurality of electrodes including a cathode sleeve, a control grid and an accelerating grid, the method of controlling the cut-off voltage which comprises assembling and insulatedly coupling the electrodes together in conventional manner but with an oversize spacing between the adjacent end of the cathode sleeve and the aperture in the control grid and while measuring the direct capacitance between the accelerating grid and cathode sleeve and while the control and accelerating grid are held fixed against movement adjusting the spacing between the cathode and control gri-d by forcing a probe against the cathode to dimple it toward the aperture in the control grid until the measuredl capacitance corresponds to the desired cut-ott value.

References Cited in the tile of this patent i UNITED STATES PATENTS 2,581,997 I Beggs Ian. 8, 1952 

